Aromatic hydrocarbon

ABSTRACT

Loss of aromatics in the raffinate removed from an extractive distillation zone is diminished by reboiling a dilute watersolvent mixture with the extractive distillation lean solvent feed.

United States Patent 11 1 Thompson 1 Oct. 2, 1973 1 AROMATIC HYDROCARBON3,537,984 11/1970 Thompson 208/325 1 4 P l 2 1 1 1mm MM L- Thomson, ParkRidge, 3333333 3/1321 6151????fffififf? 233/313 Ill. 3,560,374 2/1971Assignee: Universal P c C np ys Cl. al Des Plaines, [IL 1 I PrimaryExaminerDelbert E. Gantz l [22] ed May 1971 Assistant Examiner-C. E.Spresser [2]] App]. No.: 140,776 Attorney.lames R. Hoatson, Jr. andEdward W.

, 1 Remus [52] US. Cl. 208/313, 208/321, 208/325,

208/326, 260/674 sE [51] Int. Cl. C07c 7/08 [57] ABSTRACT [58] Field ofSearch 260/674 SE; 208/313, Loss of aromatics in the raffinate removedfrom an ex- 208/321, 325, 326 tractive distillation zone is diminishedby reboiling a dilute water-solvent mixture with the extractivedistilla- [56] Reierences Cited tion lean solvent feed.

UNITED STATES PATENTS 3,642,614 2 1972 Van Tassell 260/674 11 1 DrawF'gure PATENTEDUBT 21 1s Y 3,763,037

N VE/V 7'0 5.- Herbert L. Thompson .4 TTORNEYS AROMATIC IIYDROCARBONBACKGROUND OF THE INVENTION The present invention relates to theextractive distillation and recovery of aromatic hydrocarbons from amixture of aromatic and non-aromatic hydrocarbons. More specifically,the present invention relates to a method for controlling the amount ofaromatics lost in the non-aromatic raffinate product removed from anaromatic selective, extractive distillation, particularly when sulfolaneis utilized as the aromatic selective solvent.

Conventional processes for the recovery of high purity aromatichydrocarbons such as benzene, toluene and xylenes (BTX) from varioushydrocarbon feedstocks including catalytic reformates, hydrogenatedpyrolysis gasolines and various selective cuts of these aromaticsources, utilize an aromatic selective solvent to facilitate the removalof the aromatics from nonaromatic hydrocarbons. In certain of theseprior art processes, a hydrocarbon feedstock is first contacted in asolvent extraction zone with a solvent composition, preferablycontaining water to enhance the solvents selectivity, which selectivelydissolves the aromatic components of the feedstock, thereby forming araffinate phase comprising one or more non-aromatic hydrocarbons and'aminor amount of aromatic hydrocarbons and an extract phase comprisingsolvent having an aromatic hydrocarbon dissolved therein. The aromatichydrocarbons contained in the extract may be recovered by distillationto yield an overhead distillate containing a portion of the extractedaromatics, an aromatic side-cut fraction and a solvent bottoms fractionwhich is recycled for reuse in the solvent extraction zone. Frequently,the solvent extraction zone extract phase is subjected to an extractivedistillation (i.e., extractive stripping) to remove contaminatingamounts of non-aromatic hydrocarbons which may be contained in theextract in order to make possible the recovery of nitration gradearomatic hydrocarbons.

Also not infrequently, particularly when processing a narrow boilingrange or carbon number range mixture of aromatic and non-aromatichydrocarbons, the aromatic hydrocarbons are recovered by passing thearomatic feedstock directly to an extractive distillation zone ratherthan going through the preliminary liquid phase solvent extraction. Inthis mode of operation, a hydrocarbon feedstock and a solventcomposition which selectively dissolves the aromatic components, arecontacted, thereby forming an aromatic-rich extract stream and anon-aromatic containing raffinate overhead. To maximize the amount ofaromatic hydro- I carbons recovered from a given feedstock, it isnecessary to substantially eliminate the passage of any aromatichydrocarbons overhead in admixture with the non-aromatic raffinate.Further, when utilizing an aromatic selective solvent containing water,aromatic selective solvent is often removed overhead in admixture withthe non-aromatic raffinate. These losses of aromatic selective solventare controlled by water-washing the non-aromatic raffinate therebyremoving. the sol" 2, ered and recycled back to the extractivedistillation zone.

Accordingly, it is seen that the art has methods developed forrecovering solvent removed overhead in the non-aromatic raffinate streamproduced in the extrac tive distillation zone. However, since aromaticand non-aromatic hydrocarbons are miscible and cannot be removed bywater-wash, those aromatic hydrocarbons which are inadvertently removedoverhead in admixture with the raffinate streams are often lost in theprocess and ultimately lower the overall efficiency of a givenextractive distillation process.

SUMMARY OF THE INVENTION It is an object of this invention to provide aprocess for the recovery of aromatic hydrocarbons from a mixture ofaromatic and non-aromatic hydrocarbons by an improved, more facile andeconomical utilization of extractive distillation.

It is a more limited object of the present invention to provide a methodfor controlling the amounts of aromatic hydrocarbon which areundesirably removed in the non-aromatic raffinate product recovered fromthe extractive distillation zone, particularly when sulfolane solvent isutilized therein as the aromatic selective sol vent.

In a broad embodiment, therefore, the present invention relates to animprovement in a process for the separation of aromatic hydrocarbons:from a mixture of aromatic and non-aromatic,hydrocarbons wherein a lean,aromatic selective solvent, recovered from a solvent recoveryfractionation column, is contacted with the hydrocarbon mixture in anextractive distillation zone to produce an aromatic-rich extract streamand a nonaromatic raffinate overhead and water is separated from arelatively dilute solvent containing water stream in a water strippingzone. The particular improvement involves controlling the amount ofaromatic hydrocarbons removed in the non-aromatic raffinate by reboilingthe water stripping zone, maintained at a pressure sufficient tovaporize at least a portion of the water passed thereto, with at least aportion of the lean solvent recovered from the solvent recovery zone,thereby cooling the lean solvent. At least a portion of the thus cooledsolvent is passed to an upper portion of the extractive distillationzone. In a more limited embodiment, aromatic losses in the raffilnateoverhead may be further controlled by lowering the pressure maintainedon the water stripping zone when the amount of aromatic hydrocarbonsremoved overhead in the nonaromatic raffinate exceeds a predeterminedlevel, thereby further cooling the lean solvent which lowers the amountof aromatic hydrocarbons removed in the overhead raffinate. This controlmethod finds particular applicability when utilized in a sulfolane typesolvent process or a process utilizing N-methyl-Z- pyrrolidone assolvent. r

In a further, more limited embodiment, the nonaromatic raffinatecomprising hydrocarbons, solvent and water is removed in a vapor phaseand condensed,

' with at least a portion of the condensed water being passed as refluxinto an upper portion of the extractive distillation zone. Preferably,this water reflux is utilized only when the amount of aromatichydrocarbons re- 1 moved in the raffinate exceeds a pre-determinedlevel.

a process for separating non-aromatic hydrocarbons from aromatichydrocarbons by extractive distillation. This novel method involvesintroducing a hydrocarbon feedstock containing aromatic and non-aromatichydrocarbons into a sulfolane type extractive distillation zonemaintained under extractive distillation conditions including thepassing of a hereinafter specified lean solvent stream containing waterinto an upper portion of the extractive distillation zone to provide anaromatic, rich liquid extract stream relatively free of nonaromatichydrocarbons and comprising sulfolane, aromatic hydrocarbons and water,and a vaporous rafflnate stream comprising non-aromatic hydrocarbons,water, a minor amount of sulfolane and a predetermined amount ofaromatic hydrocarbons. The overhead raffinate stream is condensed toprovide a first aqueous stream containing a minor amount of sulfolaneand a hydrocarbon raffinate containing a minor amount of sulfolane. Thehydrocarbon raffinate stream is washed with water to provide arelatively sulfolanefree, non-aromatic raffinate product and a secondaqueous stream containing a minor amount of sulfolane. A portion of thefirst aqueous stream is passed as reflux into the upper portion of theextractive distillation zone, preferably only when the aromaticscontained in the raffmate overhead exceeds a predetermined value. Theliquid extract is passed into a solvent recovery zone maintained undersolvent stripping conditions including the introduction of steam into alower section of the recovery zone to provide a relatively non-aromaticand sulfolane free extract product and a lean solvent stream containingwater at an elevated temperature, typically of about 250F to about 400F.The remaining portion of the first aqueous stream and the second aqueousstream are passed to a water stripping zone maintained at a temperatureand pressure sufficient to provide a water vapor stream comprisingwater. This vapor stream is then condensed in admixture with thepreviously mentioned vaporous raffinate stream. The water stripping zoneis reboiled with at least a portion of the lean solvent stream withdrawnfrom the solvent recovery zone at an elevated temperature to provide acooled lean solvent stream which is then passed to the extractivedistillation zone as the previously specified sulfolane stream. in amore limited embodiment, aromatic losses in the raftinate overhead arefurther controlled by lowering the pressure maintained in the waterstripping zone when the amount of aromatic hydrocarbons contained in theraffinate exceeds the predetermined level. Lowering the pressure causesfurther cooling of the lean solvent passed to the extractivedistillation zone thereby inducing greater amounts of internal refluxwithin the extractive distillation zone thereby lowering the amountofaromatic hydrocarbons removed overhead.

in a more limited embodiment the extract removed overhead from thesolvent recovery zone is a vapor stream comprising aromatic hydrocarbonsand water relatively free of sulfolane. This vapor overhead is condcnsedand the water recovered is utilized as the water with which thesulfolane containing raffinate is washed.

Other objects, embodiments and a more detailed description of theforegoing embodiments will be found in the following, more detaileddescription of the present invention.

DETAILED DESCRlPTlON OF THE INVENTION Feedstocks suitable for use in thepractice of this invention include hydrocarbon fluid mixtures having asufficiently high concentration of aromatic hydrocarbons to economicallyjustify recovery of these aromatic hydrocarbons as a product stream. Thepresent invention is particularly applicable to hydrocarbon feedmixtures which contain at least 25 percent by weight aromatichydrocarbons, although hydrocarbon feed mixtures containing as little asl5 percent by weight aromatics and as high as 95 percent by weightaromatic hydrocarbons are suitable. A suitable carbon number range forthe feedstock is from six carbon atoms per molecule to about 20 carbonatoms per molecule, and preferably from about six to 10 carbon atoms permolecule. A preferred feedstock source is the debutanized ordepentanized effluent from a conventional catalytic reforming unit.Another satisfactory feedstock is the liquid by-product from a pyrolysisgasoline unit which has been hydrotreated to saturate olefins anddiolefins, thereby producing an aromatic hydrocarbon concentratesuitable for processing by extractive distillation techniques. A typicalfeedstock from a catalytic reforming process unit contains single ringaromatic hydrocarbons of a wide boiling mixture including benzene,toluene and xylenes in admixture with the corresponding boiling rangeparaffms and naphthenes. Thus,- it is these single ring aromatichydrocarbons whichare to be recovered from the preferred catalyticreforming reactor effluent. It is to'be pointed out, however, while thepresent invention is directed specifically to the extractivedistillation of a hydrocarbon mixture, included within the broad scopeof this invention are hydrocarbon mixtures in admixture with aromaticselective solvents such as a solvent-aromatic mixture containingnon-aromatic hydrocarbons which is produced, as extract, in a liquidphase solvent extraction zone.

Solvents included within the scope of the present invention are thosesolvents which exhibit a tendency to selectively dissolve aromatichydrocarbons when the aromatic hydrocarbons are contained in anonaromatic mixture. The preferred solvent is a solvent of the sulfolanetype. This solvent is a known article of commerce, particularly wellknown to those skilled in the art of aromatic recovery by selectivesolvent extraction and typically possesses a five-member ring containingone atom of sulfur and four atoms of carbon with two oxygen atoms bondedto the sulfur atom of the ring.

Other solvents which have a high selectivity for separating aromatichydrocarbons from non-aromatic hydrocarbons included in the practice ofthis invention are the sulfolenes such as 2-sulfolene and 3-sulfolene,2-methylsulfolane, 2-4-dimethyl-sulfolane, methyl-2- sulfonylether,n-aryl-3-sulfonylamine, 2-sulfonylacetate, diethyleneglycol, variouspolyethyleneglycols, dipropyleneglycol, various polypropyleneglycols,dimethylsulfoxide, N-methylpyrrolidone, glycol-amines,N-methyl-2-pyrrolidone, a N-methyl-2-pyrrolidoneethylene glycol mixture,and various mixtures of the above. As indicated, the specificallypreferred solvent for use in the practice of the present invention issulfolane. Also preferred is a N-methyl-Z-pyrrolidone mixture.

Since the aromatic selectivity of a solvent can be enhanced by theaddition of water to the solvent, the pret'crrcd solvent utilized in thepractice of this invention contains small quantities of water in orderto increase the selectivity of the overall solvent phase for aromatichydrocarbons over non-aromatic hydrocarbons without substantiallyreducing the solubility of the solvent phase for aromatics. The presenceof water in the solvent composition also provides a relatively volatilematerial which is distilled from the solvent in the extractivedistillation zone which is more fully discussed hereinafter, to helpvaporize the last traces of non-aromatic hydrocarbons from thehydrocarbon stream by steam distillation. The composition of the presentinvention preferably contains from about 0.5 percent to about 20 percentby weight water, in particular from about 2 percent to about percent byweight depending upon the particular solvent utilized and the processconditions at which the extractive distillation zone is operated.

Extractive distillation conditions and techniques are generally wellknown to those trained in the art and vary depending upon the particulararomatic selective solvent utilized. Therefore, for brevity, thediscussion will be generally limited to a sulfolane type solvent,particularly sulfolane, and no intent is made thereby to unduly limitthe scope of this invention to this solvent species.

Broadly speaking, extractive distillation conditions for extractingaromatic hydrocarbons from nonaromatic hydrocarbons include lowpressures and sufficiently high temperatures in order to vaporize allthe non-aromatic hydrocarbons, thereby providing a predominantlynon-aromatic fraction containing nonaromatics, a minor amount ofaromatic hydrocarbons solvent, and water. It is the specific intent ofthis invention to limit the amount of aromatic hydrocarbons removed inthe non-aromatic fraction recovered in the extractive distillation step.Also, provided is a relatively non-aromatic-free bottoms fractioncontaining aromatic hydrocarbons and solvent. Typically, this bottomsfraction contains less than 1,000 ppm and prefer ably less than 500 ppmby weight non-aromatic hydrocarbons.

Typical operating conditions in the extractive distillation zone whenutilizing a sulfolane solvent, includes a pressure from about 90 mm Hgabsolute to 40 psig, an overhead temperature of about l30F to about330F, and a bottoms temperature of about I70F to about 355F. Sulfolanesolvent to feed ratios may vary from about 1:1 to about 20:l dependingon the pressure and temperature and feed composition. The preferredsolvent to feed ratio when processing a C -C range naphtha is in therange of about 2:1 to about 6:1. A more detailed definitive descriptionof the conditions, apparatus and process flows for an extractivedistillation process utilizing a sulfolane type solvent may be found inPetroleum Refiner, September 1959, vol. 38, No. 9, pages 185 through192, the teachings of which are specifically incorporated herein byreference. As indicated, the extractive distillation zone provides arich, liquid extract stream relatively free of non-aromatic hydrocarbonscomprising sulfolane solvent having dissolved therein the desiredaromatic hydrocarbons. Also provided is a vaporous raffinate streamcomprising non-aromatic hydrocarbons, water (steam) and a minor amountof sulfolane. The water is present because the lean sulfolane solventutilized in the preferred embodiments contains water to enhance itsselectivity. The vaporous overhead also contains a minor amount ofaromatic hydrocarbons which are removed overhead and which represent aloss of aromatic hydrocarbons from the process. The essence of thisinvention lies in controlling the amount of aromatics, which are lost inthis overhead stream, at a predetermined level. This level is dictatedby the economics associated with a given unit and feedstock and can varyaccordingly. In general, it is. desired to recover more than percent ofthe aromatics contained in a given feedstock. Simul' taneouslyassociated with reducing aromatic losses will be the reduction in theamount of sulfolane which is present in the raffinate overhead. In anyevent, this vaporous raffinate stream is cooled and condensed. Uponcondensation, two liquid phases are formed, a hydrocarbon phasecontaining minor amounts of sulfolane and a first aqueous phase, alsocontaining a minor amount of sulfolane. The sulfolane contained in thehydrocarbon phase is removed by scrubbing the hydrocarbon raffinate withwater by means well known to those trained in the art, including theutilization of a packed column or a rotating disc contactor. Preferablythe water utilized is provided by a sulfolane free water streamrecovered from a hereinafter described solvent recovery column. Thiswater washing provides a sulfolane-free (i.e., less than 10 ppmsolvent)raffinate product and a second aqueous stream comprising water andsulfolane. The sulfolane contained in the described first and secondaqueous stream is recovered in a water stripping zone to be describedlater.

The aromatics contained in the extract from the extractive distillationcolumn are recovered in a solvent recovery column of the variety wellknown to those trained in the solvent extraction art. This column uti'lizes steam as a bottoms input stripping medium to aid in the separationof the aromatics and solvent. Low pressures and sufficiently hightemperatures sufficient to distill the aromatic hydrocarbons overheadare utilized with the exact conditions being a function of feedstockcomposition and solvent. However, overhead pressures of about to about400 mm Hg absolute and bottoms temperatures of about 250F to about 500Fare utilized when sulfolane is the solvent. Produced is an overheadfraction of water and aromatics, free of solvent, which uponcondensation yield a final extract product. The water is recovered andis typically utilized as the water input to the raffinate wash column.

The described loss of aromatics from an extractive distillation zone isa problem long recognized by the art. The exact amount of this loss isdetermined by a compromise dictated by economics; that is, the size ofextractive distillation zone and processing conditions utilized whichare required to produce a given purity aromatics product balancedagainst the amount of aromatics to be recovered. In other words, yieldis balanced against purity. See, for example, US. Pats. Nos. 3,146,190and 3,338,824 for a detailed discussion of these particular problems.

Overhead aromatic losses in sulfolane type units is,

particularly vexatious to solve, particularly when processinghydrocarbon feedstocks containing large amounts of aromatics, i.e., 80percent by weight or more. Aromatic losses stem from trying to attaincomplete solvent miscibility on all trays of the extractive distillationzone. In conventional distillation processes, losses of higher boilingcomponents as a vapor overhead are readily controlled by refluxing aportion of the overhead after cooling and condensation. However, inextractive distillation such as the sulfolane type, refluxing of thehydrocarbon rafflnate has a tendency to cause phase formation (twoliquid phases) within the extractive distillation column, therebyupsetting the heat balance and operation of the column. This is aparticular problem when processing highly aromatic feedstocks. Onemethod which can be utilized is controlling the temperature of the leansolvent passed to the upper section of the extractive distillation zone.By choosing a proper temperature, the lean solvent entering acts also asa heat sink thereby causing an internal reflux to form. However, thistemperature must be controlled within a relatively narrow range in orderto prevent either an aromatic carryover, contamination of the extractwith non-aromatics, or the formation of different liquid phases withinthe extractive distillation zone.

In actual practice, the lean solvent utilized in the extractivedistillation zone has been recovered from the bottoms portion of asolvent recovery column at an elevated temperature, typically of about250F to about 400F. The solvent must then be cooled before it can beused in the extractive distillation zone. The utilization of air coolingor cooling water streams, however, because of variations in atmosphericconditions, cause a wide fluctuation in the degree of cooling attainedon the solvent and as a consequence, does not effectively control thearomatic losses from the column.

In the present invention, the temperature of the lean solvent passed tothe extractive distillation column is more readily controlled byreboiling the column utilized within the process to recover sulfolanesolvent from a dilute aqueous stream. In particular, at least a portionof the hot lean solvent emanating from the solvent recovery column isutilized to reboil a water stripping column wherein at least a portionof the water contained in the spent water stream from the rafflnate washcolumn and at least a portion of the water recovered from the condensedraffinate overhead stream from the extractive distillation column isvaporized to recover the sulfolane solvent contained therein. Moreparticularly, the water is converted to steam and is used as the steamstripping medium within the solvent recovery column.

Broad conditions to be utilized within the water stripping column arewell known to those trained in the art and include a temperature andpressure sufficient to vaporize at least a portion of the water.Preferably, subatmospheric pressures such as a pressure of about 300 mmHg absolute up to about 760 mm Hg absolute, and a temperature of about180F to about 250F are maintained in the bottom section of the strippingzone.

To also assist in controlling aromatic losses in the rafflnate overhead,a portion of the water recovered when the vaporous raffinate wascondensed may be refluxed into the upper portion of the extractivedistillation column. While the exact amount of water can vary accordingto feedstock and recovery sought, it is preferred that about 25 percentto about 75 percent of the condensed water be refluxed. In particular,it is preferred that this reflux be initiated only when the aromaticlosses exceed a predetermined level.

A dilute aqueous solution of sulfolane provides a relatively constantboiling mixture which, when heat exchanged against. hot, lean solvent,provides smooth temperature control on the lean solvent passed from thesolvent recovery column to the extractive distillation column. Affordedis a temperature control technique independent of ambient temperaturefluctuations since the temperature within the water stripping column isreadily maintained within a very narrow range by simply controlling thepressure on the column. Further, dilute solvent solutions are not verysensitive to composition changes. Accordingly, the temperature on thelean solvent stream is controlled by cooling at least a portion of thesolvent recovery bottoms in the water stripper reboiler. The exactdegree of cooling is readily maintained by either over cooling a portionof the lean solvent with exact control provided by admixing hot leansolvent with the cooled solvent, by varying the pressure of the waterstripping zone, or a combination thereof whenever the aromatic losses inthe raffinate overhead exceed a predetermined level.

DESCRIPTION OF THE DRAWING The present invention can be most clearlyillustrated and described by reference to the attached schematic diagramillustrating the recovery of aromatic hydrocar bons from a catalyticreformate. Of necessity, certain limitations must be present in aschematic diagram of the type presented and no intention is made therebyto limit the scope of this invention as to feedstocks, rates, operatingconditions, solvents, etc. Certain miscellaneous appurtenances includingsome valves, controls, pumps, compressors, separators, reboilers and thelike have been eliminated. Only those vessels and lines necessary for acomplete and clear understanding of the various embodiments of thisinvention are illustrated.

Referring now to the attached drawing, a C hydrocarbon feed streamderived from a catalytic reformate containing about 10 percentnon-aromatics (paraffins and naphthenes) and about percent aromatichydrocarbons, of which greater than 90 percent are C enters via line 1,is commingled with make-up or wet solvent entering via line 2 and passedvia line 3 to extractive distillation column 4. Extractive distillationcolumn 4 is of a conventional design for the extraction of aromatichydrocarbons from an aromatic nonaromatic hydrocarbon mixture whereinsulfolane is utilized as the selective solvent. More particularly,sulfolane derived at a specified temperature in a manner to be describedlater, enters the upper portion of extractive distillation column 4 vialine 5 and contacts therein the hydrocarbon feedstock entering throughline 3. Removed overhead from extractive distillation column 4 via line6 is a vaporous raflinate stream containing essentially. all of thenon-aromatic hydrocarbons which are passed to the extractivedistillation column, and a minor amount of aromatic hydrocarbons andsulfolane. The aromatics removed overhead from extractive distillationcolumn 4 lower the expected aromatic hydrocarbon recovery desired andthis amount is to be minimized. More specifically, the present inventioncontrols the amount of aromatic losses by carefully controlling thetemperature of the lean solvent feed, containing 3 mole percent water,entering line 5 in response to the amount of aromatics removed overhead.

Referring back to the rafflnate removed overhead via line 6, thisvaporous fraction is condensed in heat exchange means 7 and passed toseparation receiver 8 wherein a hydrocarbon stream containing sulfolaneis separated and removed via line 9 and a water stream, also containingsulfolane, is removed via line 11. Illustrated is a reflux stream 10which the art has utilized to control aromatic losses within extractivedistillation column 4. According to the process of this invention,non-aromatic hydrocarbons removed from separator receiver 8 are seldom,if ever, refluxed to the upper portion of extractive distillation column4 since their passage thereto may lead to separate liquid phases formingwithin the column. However, a portion of aqueous stream 11 may bewithdrawn via line 12 and passed via line to the upper portion ofextractive distillation column 4 to assist in controlling aromatic overhead losses. Preferably, this reflux is passed to column 4 only when thearomatics contained in the vaporous raftinate exceeds a predeterminedlevel which, as indicated, is a function of the aromatic recoverydesired in the entire process.

Non-aromatic hydrocarbon raffinate is passed via line 9 to the lowerportion of water wash column 24 wherein the hydrocarbon is scrubbed withwater entering via line 23, the source of which is to be describedlater. Within wash column 24 the incoming water and hydrocarboncountercurrently contact each other. and the sulfolane solvent containedwithin the aromatic hydrocarbon is removed as a dilute aqueous medium(i.e., about 1 percent sulfolane mole basis) via line 26. An essentiallysulfolane free raffinate is removed via line 25. The sulfolane containedwithin lines 26 and 11, as a dilute aqueous solution, are commingled andpassed via line 27 to water stripping zone 28 to recover the sulfolanetherein, in a manner to be described later.

Removed from the bottom portion of extractive distillation column 4 is arich sulfolane stream relatively free of non-aromatic hydrocarbons. Thisrich aromatic extract is passed to recovery column 14 for separationtherein of the aromatic hydrocarbons from the sulfolane solvent. Morespecifically, recovery column 14 is reboiled by a hot oil reboilingmeans 15 contained within liquid trap out tray 16 with stripping steamderived from a source to be described later, entering via line 37.Removed overhead from recovery column 14 is an aromatic extractrelatively free of sulfolane solvent which is passed via line 17,commingled with condensate make-up entering via line 18, and condensedin heat exchange means 19. The resultant condensate is passed toseparator receiver 20 wherein a sulfolane free aqueous stream isrecovered and removed via line 23 and passed to raffinate wash column 24as the described wash water medium. A hydrocarbon stream is removed vialine 21 with a portion thereof passed via line 22 as reflux to the upperportion of recovery column 14. More specifically, the hydrocarbonsrecovered and removed from the process via line 21 represent at least98.5 percent of the C aromatic hydrocarbons originally passed to extractdistillation column 4 via line 1.

In a typical recovery column operation, recovery column 14 is maintainedunder sub-atmospheric pressures such as an overhead pressure of 100 mmHg absolute and a bottoms pressure of about 440 mm Hg absolute. Further,hot oil reboiler is preferably maintained at a sufficient temperature sothat the film temperature of the sulfolane never exceeds 450F to avoidsulfolane decomposition within the recovery column. Lean sulfolanesolvent is removed from recovery column 14 via line 29 with a smallportion thereof removed via line 30, passed through valve 31, andcommingled with cooled sulfolane solvent 5 for reasons to be describedlater. The majority of the sulfolane solvent, typically 90 percent ofthat solvent removed from recovery column 14, is passed via line 29 toreboiler means 32. More specifically, the dilute aqueous stream in line27 is passed to water stripper 28 for recovery therein of the sulfolanesolvent removed overhead in the vaporous raffinate. Water stripper 28 istypically maintained at an overhead temperature of about 205F, anoverhead pressure of 670 mm Hg absolute, a bottoms temperature of about212F and a bottoms pressure of atmospheric pressure. Dilute, sulfolane:containing water, is removed from a lower portion of column 28 via line34 and passed to reboiler means 32 for indirect heat exchange with thehot lean sulfolane solvent. Another portion of the dilute water streamis passed via line 35 to vaporizer 33. Water stripping column 28 andvaporizer 33 are maintained at a relatively constant temperature becauseof the dilute nature of the sulfolane solution passed thereto andprovides a constant temperature source for cooling the lean sulfolanesolvent removed from recovery column 14. In a specific situation, 90percent of the lean solvent is removed via line 29 at a temperature ofabout 350F and is cooled in reboiling means 32 to a temperature of about3l8F before passage to vaporizer 33. Within vaporizer 33 the leansolvent is further cooled to a temperature of about 250F therebyvaporizing the great majority of the water and sulfolane passed to thewater stripper 28.

The lean sulfolane solvent is then passed via line 29 and commingledwith the remaining 10 percent of the lean solvent to provide a finalsulfolane feed stream which is maintained at a temperature of 260F. Whenprocessing an aromatic feedstock of the composition described,bymaintaining the lean solvent at a temperature of 260F, aromaticrecoveries of 98.5 percent are economically feasible. Water andsulfolane heated and- /or vaporized in water stripper 28, are removedvia line 36 and commingled with vaporizer 33 effluent, removed via line35 and commingled in line 37 to provide the steam stripping streamnecessary for efficient operation of recovery column 14. Removedoverhead from water stripper 28 is an aqueous vapor fraction which ispassed via line 38 and commingled with the vaporous raffinate in line 6.

The amount of aromatic hydrocarbon removed overhead from extractivedistillation column 4 can be measured by installing analyzing means, notshown, in line 6, 9 or 25. When the aromatic losses exceed apredetermined level necessary for 98.5 percent aromatic recovery, aportion of the aqueous stream recovered from the condensed raffinateoverhead can be refluxed t0 the upper portion of the extractivedistillation column 4. In addition, the lean solvent stream can befurther cooled to a temperature below the 260F indicated. Thisadditional cooling when aromatic losses exceed the predetermined value,can be accomplished by either cooling a greater portion of the leansolvent removed from recovery column 14 or by lowering the pressure onwater receiver 28 and/or the pressure in vaporizer 33 thereby inducing agreater portion of the water to be vaporized which induces furthercooling of the lean sulfolane solvent. Since the dilute sulfolanemixture processed in water stripper 28 and vaporizer 33 is a relativelyconstant boiling'mixture, this method of controlling the temperature ofthe lean solvent stream passed to the extractive distillation column ismuch more reliable than by cooling the hot lean solvent withconventional air cooling or cooling water means,

which are subject to fluctuations in ambient temperature conditions.

I claim as my invention:

1. In a process for the separation of aromatic hydrocarbons from amixture of aromatic and non-aromatic hydrocarbons wherein a lean,aromatic selective solvent recovered from a solvent recoveryfractionation column is contacted with said mixture in an extractivedistillation zone to produce an aromatic rich extract stream and anon-aromatic raffinate overhead, and water is separated from arelatively dilute water stream containing solvent in a water strippingzone, the improved method of controlling the amount of aromatichydrocarbons removed in the non-aromatic rafflnate which comprises thesteps of:

i. reboiling the water stripping zone maintained at a pressuresufficient to vaporize at least a portion of the water passed theretowith at least a portion of the lean solvent recovered from the solventrecovery zone thereby cooling said lean solvent;

ii. passing at least a portion of the cooled solvent to an upper portionof the extractive distillation zone; and

iii. lowering the pressure in the water stripping zone when the amountof aromatic hydrocarbons removed overhead in the non-aromatic raffinateexceeds a predetermined level thereby further cooling the lean solventand lowering the amount of aromatic hydrocarbons removed overhead in theraffinate.

2. The method of claim 1 wherein said selective solvent comprises asulfolane type solvent.

3. The method of claim 1 wherein said selective solvent comprisessulfolane.

4. The method of claim 1 wherein said selective solvent comprisesN-methyl-2-pyrrolidone.

5. The method of claim 1 wherein said non-aromatic raffinate compriseshydrocarbons, solvent and water in a vapor phase and is condensed, acondensed water phase recovered and at least a portion of the waterphase is passed, as reflux, into an upper portion of the extractivedistillation zone.

6. The method of claim 5 wherein said reflux is utilized only when theamount of aromatic hydrocarbons removed in the raffinate exceeds apredetermined level.

7. A method for controlling aromatic losses from a process forseparating non-aromatic and aromatic hydrocarbons by extractivedistillation which comprises the steps of:

i. introducing a hydrocarbon feedstock containing aromatic andnon-aromatic hydrocarbons into a sulfolane type extractive distillationzone maintained under extractive distillation conditions including thepassing of a hereinafter specified lean sulfolane stream containingwater into an upper portion of the extractive distillation zone toprovide a rich liquid extract stream relatively free of non-aromatichydrocarbons comprising sulfolane and aromatic hydrocarbons and avaporous raftinate stream comprising non-aromatic hydrocarbons, water, aminor amount of sulfolane and a predetermined amount of aromatichydrocarbons;

ii. condensing the raffinate stream to provide a first aqueous streamcontaing a minor amount of sulfolane and a hydrocarbon raffinate streamcontaining a minor amount of sulfolane;

iii. washing said hydrocarbon rafflnate stream with water to provide arelatively sulfolane free raffimate product and a second aqueous streamcontain a minor amount of sulfolane;

iv. passing a portion of the first aqueous stream as reflux into theupper portion of the extractive distillation zone;

v. passing the liquid extract into a solvent recovery zone maintainedunder solvent stripping conditions including the introduction of steaminto a lower section of the recovery zone to provide a relativelynon-aromatic and sulfolane free extract product and a lean sulfolanestream containing water at an elevated temperature;

vi. passing the second aqueous stream and the remaining portion of thefirst aqueous stream into a water stripping zone maintained under atemperature and pressure sufficient to provide a water vapor streamcomprising water;

vii. condensing the water vapor stream in admixture with the raffinatestream in step (ii).

viii. reboiling the water stripping zone with at least a portion of thelean solvent stream to provide a cooled lean solvent stream; and

ix. passing at least a portion of the cooled solvent stream to theextractive distillation zone as said specified sulfolane stream.

8. The method of claim 7 wherein the pressure maintained on the waterstripping zone is lowered when the amount of aromatic hydrocarbonscontained in the rat'- finate exceeds the predetermined level.

9. The method of claim 7 wherein the reflux of step (iv) is effectedonly when the aromatic content in the raffinate exceeds a predeterminedlevel.

10. The method of Claim 7 wherein the lean sulfolane stream of step (v)is at a temperature of about 250F to about 400F.

11. The method of claim 7 wherein the sulfolane free extract of step (v)is a vapor overhead relatively free of non-aromatic hydrocarbons andsulfolane comprising aromatics and water is condensed and separated toprovide the water utilized inv washing the raffinate stream in step(iii).

2. The method of claim 1 wherein said selective solvent comprises asulfolane type solvent.
 3. The method of claim 1 wherein said selectivesolvent comprises sulfolane.
 4. The method of claim 1 wherein saidselective solvent comprises N-methyl-2-pyrrolidone.
 5. The method ofclaim 1 wherein said non-aromatic raffinate comprises hydrocarbons,solvent and water in a vapor phase and is condensed, a condensed waterphase recovered and at least a portion of the water phase is passed, asreflux, into an upper portion of the extractive distillation zone. 6.The method of claim 5 wherein said reflux is utilized only when theamount of aromatic hydrocarbons removed in the raffinate exceeds apredetermined level.
 7. A method for controlling aromatic losses from aprocess for separating non-aromatic and aromatic hydrocarbons byextractive distillation which comprises the steps of: i. introducing ahydrocarbon feedstock containing aromatic and non-aromatic hydrocarbonsinto a sulfolane type extractive distillation zone maintained underextractive distillation conditions including the passing of ahereinafter specified lean sulfolane stream containing water into anupper portion of the extractive distillation zone to provide a richliquid extract stream relatively free of non-aromatic hydrocarbonscomprising sulfolane and aromatic hydrocarbons and a vaporous raffinatestream comprising non-aromatic hydrocarbons, water, a minor amount ofsulfolane and a predetermined amount of aromatic hydrocarbons; ii.condensing the raffinate stream to provide a first aqueous streamcontaing a minor amount of sulfolane and a hydrocarbon raffinate streamcontaining a minor amount of sulfolane; iii. washing said hydrocarbonraffinate stream with water to provide a relatively sulfolane freeraffinate product and a second aqueous stream contain a minor amount ofsulfolane; iv. passing a portion of the first aqueous stream as refluxinto the upper portion of the extractive distillation zone; v. passingthe liquid extract into a solvent recovery zone maintained under solventstripping conditions including the introduction of steam into a lowersection of the recovery zone to provide a relatively non-aromatic andsulfolane free extract product and a lean sulfolane stream containingwater at an elevated temperature; vi. passing the second aqueous streamand the remaining portion of the first aqueous stream into a waterstrIpping zone maintained under a temperature and pressure sufficient toprovide a water vapor stream comprising water; vii. condensing the watervapor stream in admixture with the raffinate stream in step (ii). viii.reboiling the water stripping zone with at least a portion of the leansolvent stream to provide a cooled lean solvent stream; and ix. passingat least a portion of the cooled solvent stream to the extractivedistillation zone as said specified sulfolane stream.
 8. The method ofclaim 7 wherein the pressure maintained on the water stripping zone islowered when the amount of aromatic hydrocarbons contained in theraffinate exceeds the predetermined level.
 9. The method of claim 7wherein the reflux of step (iv) is effected only when the aromaticcontent in the raffinate exceeds a predetermined level.
 10. The methodof Claim 7 wherein the lean sulfolane stream of step (v) is at atemperature of about 250*F to about 400*F.
 11. The method of claim 7wherein the sulfolane free extract of step (v) is a vapor overheadrelatively free of non-aromatic hydrocarbons and sulfolane comprisingaromatics and water is condensed and separated to provide the waterutilized in washing the raffinate stream in step (iii).